Cathode-ray tube arrangement



Oct. 10, 1961 O GRAY 3,004,186

CATHODE-RAY TUBE ARRANGEMENT Filed Sept. 17, 1958 Fl LAMENT POWER vmeo .4 -BLANKING I PULSE IIVVEIVTOR Richard 0. Gray {H BLANKING 4 ATTORNEY 3,004,186 CATHODE-RAY TUBE GENIENT Richard 0. Gray, Broadview, 111., assignor to Zenith Radio Corporation, a corporation of Delaware Filed Sept. 17, 1958, Ser. No. 761,621 4 Claims. (Cl. 31515) This invention relates in general to cathode-ray type image reproducers and in particular to an arrangement for protecting the filament heater of such a device from inadvertent high voltage transients.

The conventional television picture tube, which is probably the most familiar cathode-ray type image reproducer, employs an electron gun structure comprising a heater element and a series of axially spaced electrodes for developing and forming an electron beam and further comprising a high voltage final anode for accelerating the beam. Since final anode voltages upwards of 15,000 volts are commonplace today, the occurrence of electric arc discharges between the several electrodes of a compact gun is not infrequent. Briefly, an arc discharge may be initiated by a dielectric breakdown between the high volt-' age anode of the gun and an adjacent electrode and is generally attributable to extraneous highvoltage transients in the final anode energizing circuit, to charge build-up.

upon the electrodes, or to sharp protuberances on the electrodes which tend to support a concentrated electric field and aggravate dielectric breakdown. Subsequent to the initial dielectric breakdown the electric arc progresses between successive electrodes culminating in an are discharge between the cathode and filament. The filament, being a low impedance element, sustains a heavy discharge current and generally is damaged to such an extent as to render the picture tube inoperative.

Electric arc discharges can be minimized by painstaking quality control including burnishing the gun electrodes to eliminate sharp protuberances and exercising extreme precaution during the assembly of the gun structure. This expedient is costly and does not constitute a completely reliable solution for preventing damage to the gun structure attributable to an electric arc discharge.

A more judicious safeguard is described in the copending application of Richard 0. Gray, Serial No. 713,209, filed February 4, 1958, and issued December 6, 1960, as Patent 2,963,617, and assigned to the same assignee as the present invention, which describes an over-voltage protective device comprising a pair of closely spaced suppressor terminals connected between a filament lead and the lead of an adjacent electrode to define a spark gap for dissipating the electric arc discharge. While this device has proven quite effective and eliminates many shortcomings of the prior art, a more economical solution, to be described below, is now available.

It is therefore an object of the invention to provide an improved arrangement for protecting the heater element of a cathode-ray type image reproducer from inadvertent high voltage transient conditions.

It is a further object of the invention to provide a simplified and economical arrangement for safeguarding the heater of a cathode-ray tube.

A cathode-ray type image reproducing arrangement constructed in accordance with the invention comprises an electron gunstructure including a cathode and a pinrality of electrodes disposed along an electron beam path for developing and accelerating an electron beam along the path. Means, including a filamentary heater disposed on the side of the cathode away from the other electrodes and having one portion maintained at reference potential, are provided for energizing the gun. While in operation the electrodes normally sustain a high interelectrode potential gradient but are subject to a transient are discharge current therebctween which tends to seek a path culminating in a disruptive discharge in the interelectrode space between the cathode and heater. The conductive strap of minimal length and resistance is'extended between one of the electrodes and the portion of the heater maintained at reference potential to constitute a low impedance path which conducts the arc discharge current around the heater-cathode interelectrode space.

The features of the invention which are. believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a fragmentary view, partly in section, of the neck portion of a cathode-ray tube, illustrating an electron gun structure;

FIGURE 2 is an end view, partly in section, of a tube socket for receiving the base portion of a cathode-ray tube;

FIGURE 3 is a schematic representation of a cathodemodulated cathode-ray tube employing the invention; and

FIGURE 4 is a schematic representation of a gridrnodnlated cathode-ray tube arrangement incorporating the subject invention.

Referring now to FIGURE 1, the neck portion of a cathode-ray type image reproducing arrangement there shown comprises an electron gun structure 10 which includes a plurality of electrodes disposed along an electron beam path for developing and accelerating an electron beam. More particularly, gun structure 10 comprises a series of axially spaced electrodes, conventionally designated G1, G2, G3, G4, and G5 which are mutually insulated for operating potentials less than predetermined breakdown voltages. That is to say, their spacing provides an insulation such that there is no breakdown in the presence of the potentials normally applied to the gun structure for developing, focusing and accelerating an electron beam. Electrodes G1-G5 are rigidly secured by the parallel glass rods 11. Means for energizing the gun structure are provided; specifically, a filamentary heater arrangement comprising the lead-ins 12 and 13 is disposed within a cylindrical cathode electrode 14. Cathode 14 is mounted within electrode G1 and is insulated therefrom by a ceramic washer 15. G1 serves as a control electrode and G2, while conventionally employed as a first accelerating electrode, also functions as a control electrode for blanking purposes. G4 comprises a focusing electrode. Electrodes G3 and G5 are conductively connected by a metallic rod 16 and comprise a final accelerating anode. G5 in turn mounts the resilient fingers 17 which axially center and support one end of electron gun structure 10. Fingers 17 include oifset portions 18 which contact an aquadag coating 19 disposed upon the inner surface of the cathode-ray tube neck. Coating 19 is connected to a source of high electric potential.

The-cathode-ray tube neck is terminated by a base structure 20 which secures a plurality of terminal pins extending into the neck portion of the tube to efiect electrical connections with assigned ones of the electron gun electrodes. More particularly, terminal pins 22, 23 and 24 are connected to electrodes G1, G2 and G4, respectively, while pins 25, 26 are connected to respective filament lead-ins 12, 13. Terminal pin 27 is electrically connected to cathode 14. A strap 30 comprising a conductive connection of minimal length and resistance extends between terminal pin 26 of filament lead 13 and pin 22 of control grid G1.

'Iheextremities of pins 22-47 opposite thatlwhich connects directly to the electrode structures terminate in the customary base pins, such as pin 50, to facilitate conductive retention by a corresponding plurality of connectors 22-27 secured within a mating tube socket 31 having a cover plate 32, see FIGURE 2. The connectors are afiixed to conductive leads which are returned to sources of energizing and biasing potential, schematically represented in FIGURES 3, 4. A shown in FIG- URE 2, a strap 33' may extend between connectors 22' and 26' to elfect a conductive connection of minimal length and resistance between pins 26, 22 and, therefore, between grid G1 and heater lead 13. Obviously, this is an alternative connection and, when employed, obviates the need for connector 30 within the tube base.

The gun structure of the cathode-ray tube of FIGURE 1 is schematically illustrated in FIGURE 3 for a condition of cathode modulation. Means for energizing gun structure 1!} are provided and include heater lead 12 which is connected to a source of filament power. Lead 13 is maintained at ground potential- Cathode 14 is connected through resistor 35 to a voltage divider network, comprising a brightness control, potentiometer 36, and a resistor 37, connected between a source of positive potential designated B+ and ground. This arrangement biases cathode 14 to a potential positive with respect to grid G1 which is maintained at ground potential by strap 30. Means are also provided for applying a video modulated signal to cathode 14 and include a blocking capacitor 38 which couples cathode 14 to a source (not shown) of video input voltage. Control grid G2 is connected to the junction of resistors 39, 40 which comprise a voltage divider network connected between 13+ and ground. A capacitor 41 couples control grid G2. to a blanldng pulse source (not shown) for gating the beam to accomplish retrace blockout in conventional manner. The focusing electrode G4 is returned to a source of positive potential B+ through a potentiometer 42, while the final anode G3G5 is connected to a source of high voltage, HV, via the conductive coating 19.

FIGURE 4 is substantially similar to the schematic representation of FIGURE 3 except that an alternative method of modulating the cathode-ray tube is employed. Specifically, a coupling condenser 43 applies a video modulated signal to control grid G1 rather than to the cathode as in the arrangement of FIGURE 3. In this embodiment, strap 31) provides a conductive connection between cathode 14 and grounded filament lead 13. Of course, for this use, strap 39 is positioned-either within the tube structure or externally thereof, as in the mating socl;etto extend between cathode 14 and heater load 13. Since cathode 14- is now maintained at ground potential, a resistor 44 returns G1 to a source of negahis potential C- through a brightness control 45 to provide a proper bias voltage between G1 and cathode 14.

In the operation of a television receiver employing the described arrangement, the several biasing and energizing potentials are applied to the cathode-ray tube for cathode modulation as per FIGURE 3 or for grid modulotion as shown in FIGURE 4. In either instance, upon the occurrence of a high voltage transient an electric arc discharge may be initiated between final anode G3G5 and adjacent electrode G2. Because of the close spacing of the gun electrodes, such an arc discharge occasions ielectric breakdowns between electrode G2 and control grid G1 and, subsequently, between electrode G1 and cathode 14. In the absence of an arc suppressor arrangement, the arc discharge culminates in a flash-over from cathode 14 to the heater filament thereby causing irreparable damage to the heater. It is to be noted however that in employing an arrangement as taught by the subject invention, conductive strap 30 provides a low impedance path to ground thus protecting the heater from are discharge. Preferably strap 39 is of minimal length and resistance in order to present the smallest impedance to the arc discharge. For this purpose the strap is connected either within the tube envelope between a grounded one of the heater filaments and the control grid, or the cathode as the case may be, or in the tube socket as illustrated in FIGURE 2. It is recognized, of course, that the conductive strap may also be connected across corresponding terminal pins on the tube base external to the envelope. By physically locating strap 30 in the described manner it may have the shortest length while making the requisite conductive connection. This is important since a strap or connector having an unnecessarily long length will present a substantial inductive reactance to the arc discharge. This obtains because of the exceedingly rapid change of current induced by the short rise time of the transient voltage. Since this inductive reactance is comparable to and frequently exceeds the impedance presented between the heater lead and the adjacent cathode surface, the heater is afiorded no protection. 7

Therefore and in accordance with the invention, a cathode-ray tube arrangement employing a conductive strap of minimal length and resistance between one of the heater lead-ins and one of the gun electrodes provides a low impedance arc discharge path and protects a cathode-ray tube heater from high voltage transient conditions. In general, the connection 30 may extend from the grounded filament lead to any of the gun electrodes to protect the heater against arc discharge but,

as a practical matter, it is usually necessary to connect it with the first grid or cathode, as described. This protection is simply and economically achieved and may be availed of either during the manufacture of the cathoderay tube or thereafter.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A cathode-ray tube comprising: an electron gun structure including a cathode and a plurality of other electrodes disposed along an electron-beam path for developing and accelerating an electron beam along said path; means for energizing said gun structure including a filamentary heater disposed on the side of said cathode away from said other electrodes and having one portion thereof maintained at a reference potential, said electrodes in operation normally sustaining a high interelectrode potential gradient but subject to transient are discharge current therebetween tending to seek a path culminating in a disruptive discharge from said cathode to said heater in the interelectrode space therebetween; and a conductive strap of minimal length and resistance extending between one of said electrodes and said one portion of said heater to constitute a low impedance path conductive of said are discharge current around the.

heater-cathode interelectrode space.

2. A cathode-ray type image reproducing arrangement comprising: an electron gun structure having a cathode, an intensity control grid, and other electrodes disposed along an electron beam path for developing and accelerating an electron beam along said path; means for energizing said gun structure including a filamentary heater disposed on the side of said cathode away from said intensity control grid and other electrodes and having one portion maintained at a reference potential, said electrodes in operation normally sustaining a high interelectrode potential gradient but subject to transient arc discharge current therebetween tending to seek a path culminating in a disruptive discharge from said cathode to said heater in the interelectrode space therebetween; means for applying a video modulated signal to said cathode; and a conductive strap of minimal length and resistance extending between said intensity control grid and said one portion of said heater to constitute a low impedance path conductive of said are discharge current around the heater-cathode interelectrode space.

3. A cathode-ray type image reproducing arrangement comprising: an electron gun structure having a cathode, an intensity control grid and other electrodes disposed along an electron beam path for developing and accelerating an electron beam along said path; means for energizing said gun structure including a filamentary heater disposed on the side of said cathode away from said intensity control grid and other electrodes and having one portion maintained at a reference potential, said electrodes in operation normally sustaning a high interelectrode potential gradient but subject to transient arc discharge current therebetween tending to seek a path culminating in a disruptive discharge from said cathode to said heater in the interelectrode space therebetween; means for applying a video modulated signal to said intensity control grid; and a conductive strap of minimal length and resistance extending between said cathode and said one portion of said heater to constitute a low impedance path conductive of said are discharge current around the heater-cathode interelectrode space.

4. A cathode-ray type image reproducing arrangement comprising: an electrongunstructmesinelndingnaeatln n.

ode and a plurality of other electrodes disposed along an electron beam path for developing and accelerating an electron beam along said path; means for energizing said gun structure including a filamentary heater disposed on the side of said cathode away from said other electrodes,

said electrodes in operation normally sustaining a high interelectrode potential gradient but subject to transient arc discharge current therebetween tending to seek a path culminating in a disruptive discharge from said cathode to said heater in the interelectrode space therebetween; a base structure including a plurality of terminal pins conductively connected to respective difierent ones of said electrodes and to said heater; a socket having a corresponding plurality of connectors for conductively receiving said terminal pins and having one of said heater connectors maintained at a reference potential; and a conductive strap of minimal length and resistance extending between a connector associated with one of said electrodes and said one connector associated with said heater to constitute a low impedance path conductive of said are discharge current around the heater-cathode interelectrode space.

References Cited in the file of this patent 

